This invention relates to an improvement in a method for curing a curable rubber composition composed of a halogen-containing rubber and a member selected from the group consisting of a vulcanizing agent and a vulcanization accelerator other than thiazole-type vulcanization accelerators which comprises heating the composition in the presence of an acid acceptor. Specifically, it relates to a method for curing a curable halogen-containing rubber composition which can give a cured product having improved water resistance, chemical resistance, insulating property, etc.
More specifically, this invention pertains to an improved method which comprises heating the aforesaid curable rubber composition in the presence of, as an acid acceptor, a calcinated product of a hydrotalcite compound represented by the following formula (1) EQU M.sub.x.sup.2+ Al.sub.2 (OH).sub.2x+6-nz (A.sup.n-).sub.z.mH.sub.2 O (1)
wherein M.sup.2+ represents at least one metal element selected from the group consisting of Mg and Zn, A.sup.n- represents an anion having a valence of n, m represents a positive number, and x and z represent positive numbers satisfying the following expressions EQU 12&gt;x.gtoreq.2 EQU z.ltoreq.2,
thereby giving a cured product having improved water resistance.
Halogen-containing rubbers such as chlorinated polyethylene rubber, polychloroprene rubber and chlorinated butyl rubber are utilized as rubbers having particularly excellent weatherability and chemical resistance. Curing of the halogen-containing rubbers requires the use of an acid acceptor in addition to a vulcanization accelerator in order to cure them. Heretofore, magnesium hydroxide, calcium hydroxide, lead oxide, and other oxides, hydroxides, carbonates, etc. of elements of Groups II and IVa of the periodic table have been used as acid acceptors for this purpose.
Use of these acid acceptors, however, has various difficulties. For example, the use of compounds of elements of Group II of the periodic table as acid acceptors has the defect that the resulting cured rubber product has poor water resistance. This is presumably because strongly hygroscopic halides and/or basic halogen compounds of these elements are formed. Since poor water resistance at the same time means reduced insulating properties of the cured rubber product, lead compounds such as lead oxide have to be used as acid acceptors for uses which strongly require water resistance and chemical resistance.
The use of lead-containing acid acceptors, however, has the disadvantage that it gives cured products of poor mechanical strength and heat resistance because of the slow speed of vulcanization and the low degree of vulcanization, and in addition, the lead-containing acid acceptors have toxicity.